Why the need to double the height anyway.?
On a different note i have made maps in flame that are 500 x 500 they work in game with a small code change.
That was fun bigger maps
But i cant see a need for height.

I was totally dreaming, like literally sleeping and seeing things, about epic heightmaps with huge cliffs and slopes. Also skyscrapers [1][2][3].

I think i also have managed to double the height multiplier in the game to see how it looks if we get epic heights, but i don't think i made any screenshots. It used to be actually higher in master for some time, but then it was rolled back before one of the releases. Like, you don't really need higher precision, you only need higher maximum value, so your idea can be implemented by adding a per-map height multiplier in a better map format (as feature request for #4291).

One of the tricky problems would be to avoid the camera digging into the ground on high slopes. It doesn't hurt that much currently, but quickly gets worse when the heights increase. But apart from that, and map format, i think it's all it takes.